1. Field of the Invention
The invention relates to a developing apparatus for developing a photosensitive member and having a cleaning function of removing a toner residual on the photosensitive member, and an image forming apparatus provided with this developing apparatus.
2. Description of Related Art
In image forming apparatuses such as a copying machine, a printer and a facsimile apparatus adopting an electrophotographic process, and a compound machine of these there is a case where in order to collect any untransferred toner not transferred to a transfer medium but residual on a photosensitive member, the untransferred toner is collected by a so-called “cleanerless method” of collecting the untransferred toner by a developing device without using a cleaner apparatus contacting with the photosensitive member.
When this cleanerless method is used, the untransferred toner can be recycled without being disused. Also, a frictional member does not contact with the photosensitive member and therefore, the abrasion of the photosensitive member can be prevented. Accordingly, a running cost can be reduced.
Particularly, when a two-component developing method using a developer consisting of a mixture of a toner and a magnetic carrier is used as a developing method, collectability of the untransferred toner by a magnetic brush and the re-chargeability of the collected toner are improved. Therefore, the two-component developing method is suited for the cleanerless method.
The following developing apparatus is known as an example of a developing apparatus adopting the cleanerless method.
That is, a cleaning and developing apparatus having at least two rollers disposed in opposed relationship with a photosensitive member, and using a two-component developer, wherein a roller disposed upstream with respect to the rotation direction of the photosensitive member is a cleaning roller for cleaning the photosensitive member, and a roller disposed downstream of the cleaning roller with respect to the rotation direction of the photosensitive member is a developing roller for developing an electrostatic latent image on the photosensitive member with the two-component developer (see Claim 1 and FIG. 3 of Japanese Patent Application Laid-open No. H08-123196).
This developing apparatus has disposed therein the roller given a cleaning function and the roller given a developing function, to thereby make the collectability of the untransferred toner and the developing property which are important in the cleanerless method compatible.
The developing apparatus, as shown in FIG. 7 of the accompanying drawings, makes a cleaning roller 11 and a developing roller 12 carry a two-component developer thereon, and applies predetermined biases (DCVB1+AC) and (DCVB2, respectively, thereto and therefore, is of a construction of a so-called “multi-stage developing type” which develops a latent image on a photosensitive drum 1 by the use of virtually two developing rollers.
However, the heretofore proposed cleanerless methods including the above-described conventional example have suffers from the following problems.
When a formed image includes many portions of high density, the amount of toner consumed by a developing device becomes great. Therefore, usually in the two-component developing method, a toner corresponding to the consumed amount is adapted to be sequentially supplied and thus, the amount of toner supply has also been increased. In such a case, the imparting of charges to the toner by the agitation of the two-component developer is liable to become insufficient, and in some cases, the fogging characteristic itself of the developing device has been somewhat aggravated. The aggravation of the fogging characteristic increases a fogged toner on the photosensitive member and also induces a reduction in cleaning performance.
Also, in the case of the cleanerless method having no cleaner member contacting with the photosensitive member, an increase in the toner not removed and the fogged toner has in some cases led to the vicious circle that the hindrance of charging is caused during the charging of the photosensitive member to thereby cause a further increase in fogging. Particularly, when image formation including many portions of high density has been continuously effected, such a vicious circle has often been caused.